1. Field of the Invention
The present invention relates to a heat dissipator including a coolant passage, and more particularly, it relates to a heat dissipator which is preferably emloyable as a cooling substrate for a semiconductor device generating a large quantity of heat or a transmission window for high-energy light.
2. Description of the Prior Art
In response to the increasing requirement for longdistance transmission or the like, the output power of a semiconductor laser employed for optical communication is increased. As a consequence, the heat generated from a semiconductor element itself is also increased. Further, improvement of performance and miniaturization of a portable information system device by results in an increase of the generated heat per unit surface area in a semiconductor element included in the device.
At present, it becomes a problem that the heat radiability of a material such as alumina, which is mainly applied to a radiating substrate for such a semiconductor element, is insufficient in case of packaging the aforementioned semiconductor element of high performance. Namely, the specific heat conduction resistance of the conventional radiating substrate itself is too high to sufficiently dissipate heat generated from the semiconductor element to the ambient environment. In order to solve this problem, it is effective to employ a material having higher heat conductivity in place of the conventional material such as alumina. It is now being started to use diamond, which has; the highest heat conductivity among the existing materials, for a radiating substrate for a semiconductor laser diode or the like.
Requirements for various optical windows as used in a large-scale synchrotron radiation experimental apparatus or the like, tend to become more strict with respect to intensity of light to be transmitted and environment where the windows are to be used. Thus, it is predicted that requirements for various physical properties such as the mechanical strength of the window, endurance for radiation and the like hereafter become more strict.
While Be, Si, ZnSe, NaCl and the like are employed as the materials for various types of optical windows, these window materials generally have very small heat conductivities. If windows made of such materials are irradiated with high-energy light, therefore, the temperatures of the irradiated surfaces of the windows themselves are increased to cause problems such as fusion, alteration and breakage. Thus, the light energy levels allowing employment of these windows are restricted.
On the other hand, diamond, which can transmit light over an extremely wide range covering vacuum ultraviolet rays, visible rays and infrared rays, serves as a superior optical window material. In general, however, a window formed of diamond can be cooled only from its periphery. Thus, it is predicted that the diamond window is insufficiently cooled as its size is increased.
Heat transported by the radiating substrate must be finally transmitted to the external air or water, to be dissipated. Similarly, heat generated from the window must also be finally dissipated to the external air or water. As to both of the radiation substrate and the window, heat-conductive materials must be used as thermal loads applied thereto are increased as a matter of course, while the conducted heat must be efficiently removed from the substrate and the window. To this end, a fin or a coolant pipe is mounted on the rear surface of the heat radiation substrate, to increase the radiating area and the radiation efficiency. However, additional heat conduction resistance is inevitably caused on the mounting portion if the cooling pipe is mounted on the rear surface of the substrate, while the fin is insufficient in cooling efficiency. In case of the window, on the other hand, a cooling pipe mounted on its rear surface results in a problem in compatibility with light transmittance.
Japanese Patent Laying-Open No. 4-273466 (1992) proposes a structure of feeding a coolant to a three-dimensional integrated circuit board formed of stacked diamond substrates by providing holes passing through the circuit board along its thickness in side edge portions thereof. In this structure, however, a portion around the center of the board, i.e. that predicted to be most increased in temperature in practice, is farthest from the holes for passing the coolant, to result in inferior heat dissipativity.
Each of Japanese Patent Laying-Open Nos. 8-227953 (1996) and 8-227956 (1996) discloses a cooling substrate including passages for coolant in parallel with the substrate surface and a method of fabricating the same. In fabrication of this cooling substrate, coolant grooves are formed on a major surface of a thin plate (e.g., a diamond thin plate) of a heat-conductive material having heat conductivity of at least 10 W/cm.multidot.K by working with a laser beam, selective vapor deposition or selective etching. The surface of the heat-conductive thin plate provided with the coolant grooves is bonded to a base material, thereby obtaining the cooling substrate. The heat-conductive thin plate provided with the coolant grooves can alternatively be bonded to another heat-conductive thin plate. However, an adhesive intervenes in such a cooling substrate and it reduces the heat conductivity or light transmittance of the cooling substrate. Further, it is likely that the adhesive flows into the coolant grooves during the bonding step, reducing the product yield. In case of forming the coolant grooves by selective vapor deposition or selective etching, further, it takes time to form and remove a mask.
Each of Japanese Patent Laying-Open Nos. 8-293573 (1996) and 8-325097 (1996) proposes a micro cooling device having a passage structure for passing coolant therein, a complex device for electronic components and methods of fabricating the same. The passage structure is formed with a substrate having linear cavities and a cover layer for covering these cavities relative to the exterior. The cover layer is electrically insulative and heat-conductive, and a vapor-deposited diamond layer is concretely proposed therefor. In such prior art, however, it is likely that the cover layer enters the cavities, filling the same. Further, most parts of inner walls of the passages for the coolant have low heat conductivity and high heat conduction resistance. In addition, nothing is mentioned as to light transmittance of the substrate.
J. Electrochem. Soc., Vol. 138, 1991, pp. 1706-1709 discloses a diamond heat sink having microchannels for passing coolant therethrough. In this diamond heat sink, an SiO.sub.2 layer is formed on inner walls of the groove-shaped microchannels formed on an Si substrate, for suppressing diamond growth thereon, thereby preventing the microchannels from being filled. It is described that the microchannels can be substantially covered with diamond layers by continuing diamond synthesis for about 80 hours. However, it is likely that the diamond layers grown crosswise over openings of the groove-shaped microchannels cannot be completely joined to and integrated with each other, and the product yield is industrially reduced due to leakage of the coolant or the like. Also in this case, the grooves are formed on the substrate side of the coolant passages, and hence the heat sink disadvantageously has high heat conduction resistance, similarly to Japanese Patent Laying-Open No. 8-293573. In addition, no consideration is made as to light transmittance of the substrate.
As hereinabove described, the conventional radiating substrate or high-energy light window is limited in heat radiability. In the radiating substrate including coolant passages, further, improvement of the heat radiability is limited due to the heat conduction resistance of the mounting portions for the passages or due to problems in the fabrication process. In case of applying a radiating substrate including coolant passages to a window, it is difficult to obtain a window which can transmit light of a wide wavelength range over a wide area while implementing high heat radiability in the prior art.